1. Field of the Invention
This invention relates generally to current mirror circuits and, more specifically, to bipolar junction transistor (BJT) current mirrors having a large dynamic range that is independent of BJT current gain.
2. Description of the Related Art
The current mirror circuit is a popular building block for accomplishing current reproduction in integrated circuitry. The current mirror in its most basic form consists of two matched transistors with their bases and emitters connected together. In addition, one of the transistors is connected as a diode by shorting its collector to its base. The diode-connected transistor is fed by a signal current source and the mirror output current is taken from the collector of the second transistor, which is maintained in its active-mode operating region by keeping its collector voltage higher than its base voltage at all times. The current mirror circuit is one of several devices known in the integrated circuit arts for reproducing a DC current generated at one location to provide for distribution to other locations within the integrated circuit.
Although the two-transistor current mirror circuit function is independent of supply voltage, the mirror gain is a function of transistor beta (.beta.), such that the output current I.sub.o divided by the input current I.sub.i is I.sub.o /I.sub.i 1/(1+2/.beta.). The art is replete with current mirror design improvements made to increase linear dynamic range and reduce transistor beta dependency. The more complex current mirror designs provide special capabilities, such as high accuracy over many orders of current magnitude, exceptionally high output resistance, very low or very high transfer ratios, and so forth. Reference is made to the tutorial by Barrie Gilbert, "Chapter 6: Bipolar Current Mirrors", pp. 239-296, Analog IC Design: The Current Mode Approach, C. Toumazou, et al., Eds., Peter Peregrinus Ltd., London, 1990, for a detailed description of the bipolar current mirror art.
Examples of design improvements include the "Wilson" mirror circuit, which employs a third BJT in the output collector circuit to reduce performance sensitivity to transistor beta. If all three BJTs are assumed to have matched characteristics, the gain of the Wilson mirror circuit is I.sub.o /I.sub.i =1/(1+1/(.beta..sup.2 +2.beta.). However, even the Wilson mirror circuit suffers from poor linearity when implemented in BiCmos integrated circuit technology because the PNP BJT beta values are often as low as six, introducing linearity errors on the order of two percent. Although this problem can be mitigated by using NPN BJT current mirror designs, PNP designs are necessary for many applications.
Bipolar current mirrors employing field-effect transistor (FET) buffer elements to improve input and output impedance ratios can overcome this beta dependency. For instance, in U.S. Pat. No. 4,473,794, Scott H. Early et al. disclose a current repeater circuit that employs FET helper transistors in a BJT current mirror. Unfortunately, the low FET device transconductance causes such current mirror designs to suffer from large collector voltage variations that degrade mirror linearity because of varying Early voltage effects.
In U.S. Pat. No. 5,079,518, Myles H. Wakayama discloses a current mirror circuit that uses a pair of NMOS FET transistors with a BJT helper transistor coupled to control the gate voltages responsive to input current. While MOS FET current mirror circuits are useful, it is much more difficult to accurately match FET parameters than it is to match BJT parameters in BiCmos integrated circuit technology. Accordingly, there is a clearly felt need for a PNP BJT current mirror design for BiCmos technology that is entirely independent of BJT beta.
These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.